Filters of the above-mentioned type are used, i.a. for permanent or temporation implantation in the vena cava, in particular the inferior vena cava, to prevent thrombi or emboli from reaching the patient's lungs and causing pulmonary embolization.
Such filters are well known in the art, e.g. from U.S. Pat. Nos. 3,952,747, 4,425,908 and 4,619,246 as well as published European patent application No. 430848.
For the filter disclosed in U.S. Pat. No. 3,952,747 a relatively simple insertion instrument is described, comprising a single catheter to the distal end of which a filter carrier is connected by means of a connecting member, in which the filter legs are retained during the insertion which is performed by sliding the catheter with the filter arranged on the filter carrier directly through the blood vessels of the patient. When the carrier has been brought into position, a hydraulic pressure is applied through the catheter to act on a spring-biased piston inside the carrier whereby the spring-biased filter legs retained behind a ramp portion of the connecting member are released.
By such an instrument the risk of damage during insertion is relatively high due to the direct contact between the filter and filter carrier and the blood vessel. In addition, the filter carrier with the internal spring-biased piston is a rather complicated device which moreover entails the complication in operation that hydraulic pressure must be applied through the catheter.
For the filter disclosed in U.S. Pat. No. 4,425,908 insertion is performed by sliding a guide wire to the distal end of which the filter is connected through a catheter. To retain the filter legs is a collapsed state during displacement through the catheter the use of a shape-memory alloy for the filter is prescribed. The necessity of cooling equipment for cooling the filter to bring it to the collapsed state renders this method rather complicated.